In the existing large scale product design, the CB circuit board and the XB circuit board need to be designed separately, and these two circuit boards are conducted through a flexible flat cable (FFC). In this design, connectors are needed for both two ends of the XB circuit board and the CB circuit board which are conducted with an FFC therebetween. Because an FFC and an extra number of connectors are used for this design, the overall assembling process becomes more complex, finally resulting in cost increase and yield degradation.